aquarium salt. if you're not using it, start it up right away. there are several medical remedies available, but the simplest solution, and the one with the best long-term effectiveness is a pinch of aquarium salt.

is this the only pleco you have in this tank? they usually prefer to be kept in small groups (2-4 is fine).

Many plecos are aggressive and as they get larger can hurt each other badly. They lock their pectoral fins out and hit each outer. Bristle-nose plecos on the other hand will bring their gill spikes out and jab each other with them, they probably lock their pectorals too.

By only growing to their tank size, you mean stunting. Toxin levels building up in such a small space will damage their internal organs, which will keep them small. This produces side effects such as decreased life span (people who keep goldfish in bowls say they live 2 weeks average, people who take care of their fancy goldfish see a life of 15ish years). Ammonia, nitrite, and nitrate harm the fish's ability to process oxygen, the gills develop hyperplaisia which makes oxygen transfer difficult, another cause of stunting.

People often compare keeping a fish in a small tank to keeping a dog in a crate it's whole life.. but the truth is there's much more to it. The poor fish are poisoning themselves, they are the cause of their own illness, the only prevention is to allow a larger volume of water for dilution of toxins and sufficient space for bacteria to grow to help control measurable wastes such as ammonia. Toxins such as urea aren't measurable but also exist in the water, causing additional harm.

If you've had them seven months like this, a lot of damage has been done, their internal organs are badly damaged and they will never reach their potential life spans.

There's no way around this, sadly.. Your tank will constantly have an ammonia problem in such a small volume for so many fish, and the only solution is a much larger tank, depending on what exactly is in there.